Recently, as liquid crystal display devices which have wide viewing angle characteristics, transverse electric field mode devices (including IPS mode devices and FFS mode devices) and vertical alignment mode (VA mode) devices are used. The VA mode devices are superior to the transverse electric field mode devices in terms of mass production and therefore have been used in a wide variety of TV applications and mobile applications. The most popular VA mode devices are MVA mode devices. An MVA mode device is disclosed in, for example, Patent Document 1.
In the MVA mode devices, linear alignment control means extending in two mutually-orthogonal directions (slits formed in electrodes or ribs) are provided to form four liquid crystal domains between the alignment control means. The azimuthal angles of the directors which are representative of the respective liquid crystal domains are 45° relative to the polarization axes (transmission axes) of polarizers in a crossed Nicols arrangement. Supposing that the azimuthal angle of 0° is identical with the 3 o'clock direction of the clock dial and that the counterclockwise direction is the positive direction, the azimuthal angles of the directors of the four domains are 45°, 135°, 225°, and 315°. Such a structure in which four liquid crystal domains are formed in one pixel is referred to as “4-domain alignment structure” or simply “4D structure”.
For the purpose of improving the response characteristics of the MVA mode, a technology called “Polymer Sustained Alignment Technology”, also known as “PSA technology”, has been developed. The PSA technology is disclosed in, for example, Patent Documents 2 to 7. In the PSA technology, alignment sustaining layers (“polymer layers”) are formed by polymerizing, after assemblage of a liquid crystal cell, photopolymerizable monomers contained in a prepared liquid crystal material in the presence of an applied voltage across the liquid crystal layer, and the resultant alignment sustaining layers are used to cause the liquid crystal molecules to have a pretilt. By adjusting the distribution and intensity of an electric field applied during the polymerization of the monomers, the pretilt azimuths (the azimuthal angles in the substrate plane) and the pretilt angles (the elevation angles relative to the substrate plane) of the liquid crystal molecules can be controlled.
Patent Documents 3 to 7 also disclose a structure which employs a pixel electrode with a fine stripe pattern in combination with the PSA technology. In this structure, when a voltage is applied across the liquid crystal layer, liquid crystal molecules are aligned parallel to the longitudinal direction of the stripe pattern. This contrasts with the conventional MVA mode described in Patent Document 1 in which liquid crystal molecules are aligned in a direction perpendicular to a linear alignment control structure, such as slits or ribs. The line-and-space of the fine stripe pattern (hereinafter, sometimes referred to as “comb tooth-like fine electrode configuration”) may be smaller than the width of the alignment control means of the conventional MVA mode devices. Therefore, the comb tooth-like fine electrode configuration is advantageous in that it can more readily be applied to small size pixels than the conventional MVA mode alignment control means.
FIG. 12 shows a liquid crystal display device 500 of the prior art which includes a pixel electrode 512 that has a comb tooth-like fine electrode configuration. FIG. 12 is a plan view schematically showing a region corresponding to one pixel of the liquid crystal display device 500.
The pixel electrode 512 of the liquid crystal display device 500 includes, as shown in FIG. 12, a cruciform trunk portion 512a which is arranged so as to coincide with the polarization axes P1 and P2 of a pair of polarizers (not shown) which are in a crossed Nicols arrangement, a plurality of branch portions 512b extending from the trunk portion 512a in directions of approximately 45°, and a plurality of slits 512c provided between the plurality of branch portions 512b. The pixel electrode 512 is electrically coupled to a thin film transistor (TFT) 514. In FIG. 12, a top gate type TFT 514 is shown.
The TFT 514 is supplied with a scan signal from a scan line 515 and an image signal from a signal line 516. In the configuration shown in FIG. 12, the scan line 515 is provided so as to transverse the center of the pixel. The gate electrode GE of the TFT 514 is formed so as to branch off from the scan line 515. The pixel electrode 512 is electrically coupled to the drain electrode DE of the TFT 514.
As shown in FIG. 12, the liquid crystal display device 500 includes a storage capacitor line 518 and a storage capacitor electrode CSE. The storage capacitor line 518 is formed of the same conductive film as a scan line 515 (i.e., at the same level as the scan line 515). The storage capacitor electrode CSE is formed of the same conductive film as a semiconductor layer SL of the TFT 514 (i.e., at the same level as the semiconductor layer SL). The storage capacitor line 518 is supplied with a potential that is equal to that of the counter electrode (which is arranged so as to oppose the pixel electrode 512). On the other hand, the storage capacitor electrode CSE is supplied with a potential that is equal to that of the pixel electrode 512.
As schematically shown in FIG. 13, the pixel electrode 512, the counter electrode that is arranged so as to oppose the pixel electrode 512, and a liquid crystal layer interposed therebetween are configured to form a liquid crystal capacitor CLC. On the other hand, the storage capacitor electrode CSE, the storage capacitor line 518 (more strictly, part of the storage capacitor line 518 extending over the storage capacitor electrode CSE), and an insulation layer interposed therebetween are configured to form a storage capacitor CS.
When a voltage is applied between the pixel electrode 512 that has the above-described comb tooth-like fine electrode configuration and the counter electrode, an oblique electric field is produced in each slit 512c (i.e., a space where the conductive film of the pixel electrode 512 does not exist). Such an oblique electric field defines the azimuth in which the liquid crystal molecules incline (the azimuthal angle component of the long axes of the liquid crystal molecules inclined by the electric field), and four (four types of) liquid crystal domains are formed in the liquid crystal layer in each pixel. The alignment azimuths of the liquid crystal molecules in the respective ones of the four liquid crystal domains are different from one another, so that the azimuthal angle dependence of the viewing angle is decreased, and display of wide viewing angles is realized.